Port Clock functions to use Duration class (#19229)

This changes the arguments in clock functions to be `Duration` and
converts call sites and constants into `Duration`. There are still some
more functions around that should be converted (e.g.
`timeout_deferred`), but we leave that to another PR.

We also changes `.as_secs()` to return a float, as the rounding broke
things subtly. The only reason to keep it (its the same as
`timedelta.total_seconds()`) is for symmetry with `as_millis()`.

Follows on from https://github.com/element-hq/synapse/pull/19223

synapse/api/ratelimiting.py

@@ -27,6 +27,7 @@ from synapse.config.ratelimiting import RatelimitSettings
 from synapse.storage.databases.main import DataStore
 from synapse.types import Requester
 from synapse.util.clock import Clock
+from synapse.util.duration import Duration
 from synapse.util.wheel_timer import WheelTimer
 
 if TYPE_CHECKING:
@@ -100,7 +101,7 @@ class Ratelimiter:
         # and doesn't affect correctness.
         self._timer: WheelTimer[Hashable] = WheelTimer()
 
-        self.clock.looping_call(self._prune_message_counts, 15 * 1000)
+        self.clock.looping_call(self._prune_message_counts, Duration(seconds=15))
 
     def _get_key(self, requester: Requester | None, key: Hashable | None) -> Hashable:
         """Use the requester's MXID as a fallback key if no key is provided."""